The present invention relates to a repeater for a digital communication system and, more particularly, to a repeater having a channel monitoring system which can determine a fault position on a PCM communication channel.
FIGS. 1, 2, and 3 show three typical conventional channel monitoring systems of digital communication repeaters, respectively. In FIGS. 1, 2, and 3, connections other than that concerning channel monitoring are omitted.
In the repeater of the type shown in FIG. 1, a reception signal 1 is supplied from a receiver 3 to only a transmitter 4. Therefore, a monitoring unit 5 can detect interruption of the reception or transmission signal but cannot detect a synchronizing signal in the reception signal. Since a step out fault caused by a degradation in channel quality can be detected only at a terminal, a repeating section in which the channel fault occurs cannot be determined.
In the repeater of the type shown in FIG. 2, a synchronizing signal detector 6 is inserted between the receiver 3 and the monitoring unit 5. Therefore, when the channel quality of the reception signal is degraded and step out occurs, the monitoring unit 5 detects this step out. In this case, fault information can be generated in a repeating section in which the channel fault has occurred. However, since a signal including the fault is directly reproduced and transmitted, a repeating section in which the fault has occurred cannot be determined. In addition, it cannot be determined whether the fault has occurred in the synchronizing signal detector 6 or in the reception signal 1.
In the repeater of the type shown in FIG. 3, a synchronizing signal generator 7 and a signal selector 8 are added to the repeater shown in FIG. 2. When the synchronizing signal detector 6 detects a channel fault in the reception signal 1, it supplies fault information to the monitoring unit 5 and the signal selector 8. The signal selector 8 selects a signal of the synchronizing signal generator 7 in the self repeater in place of the reception signal 1 and supplies it to the transmitter 4. For this reason, even if a fault occurs in the reception signal 1, a synchronizing signal is always included in the transmission signal 2. Therefore, no channel fault information is generated after the next repeater. However, it cannot be determined whether the fault has occurred in the synchronizing signal detector 6 or in the reception signal 1.